Conventional robot mechanisms use rigid links articulated at a plurality of joints or sliding within guiding bearings relative to each other. Such mechanisms, because their links are usually loaded in a bending mode, always yield heavy robots in relation to their load carrying capability. The ratio of robot weight to its payload may range between 100 to over 1000, with the ratio increasing for robots having larger working envelopes. For example, an automotive painting robot that carries 5 kgs of payload may weigh as much as 1000 Kgs with a ratio of 200. Using the same robot for painting aircraft, for which the working envelope is particularly large, additional positioning structures and mechanisms weighing several tons must be added. Such structures are complex, heavy and costly to install, align, and maintain. Their drives consume considerable energy, they use valuable space, require special costly considerations to avoid damaging collisions with the work objects of the robots.
Some prior art has adopted tensile members such as is disclosed in U.S. Pat. No. 4,666,362, Landsberger, et al, issued May 19, 1987, wherein a passive compressive central spine is compressed against the pull forces of tensile cables. Such mechanisms are known as parallel link mechanisms, wherein motion is actuated by links that are all anchored to one common base. This is in contrast with serial type robot mechanisms wherein several links may be stacked in series with each other with one or more links anchored to a base.
Parallel link mechanisms having relatively long links offer advantages over serial link mechanisms for constructing robots with large work envelopes. Their relative component simplicity, low weight, and the application of driving forces directly to the payload, hence requiring minimum driving power, are attractive features. However, such advantages are less relevant for mechanisms providing a small working or motion envelope or are limited to orientation control such as robotic wrist actuation. It is even disadvantageous to have long links anchored to a robot base and remotely actuating the orientation function of a robot wrist, since such links would have excessive compliance and are difficult to construct with necessary accuracy by virtue of their length.
It is therefore preferable for robot construction to use a parallel link type mechanism where it has the advantage, of positioning when the work envelope is large. It may then be complemented with a simpler serial linkage for orientational adjustments, or additional short positional moves whenever additional degrees of freedom are required. Some prior art, such as that disclosed in U.S. Pat. No. 4,666,362 employs driving methods which cause the actuators to be loaded continuously against a passive central repulsive force, regardless of the motion or load of the mechanism thus consuming energy unnecessarily. It is preferable, of course, that the loading of actuators be devoted to the productive function of the mechanism and be relieved of internal loads.
Furthermore, all conventional robots, including prior art robots of the parallel link type, are built with a common base supporting the structure of the robot. Such a common base is inflexible and allows limited installation possibilities. Therefore, it is desirable to have a robot that can be mounted to fit the environment of its application such as by attaching it selectively to existing building structures by locating the supporting points favorably to minimize the loading on the robot components, increase the work envelope, and avoid interferences with surrounding equipment and structures.